The present invention relates to the design of electrochemical cells and electrochemical systems, and more particularly to electrolysers in single stack and multiple stack configurations.
Electrolysis is a method for production of a chemical reaction that is electrically driven by passage of an electric current, typically a direct current (DC), through an electrolyte between an anode electrode and a cathode electrode. An electrochemical cell is used for electrochemical reactions and comprises anode and cathode electrodes immersed in an electrolyte with the current passed between the electrodes from an external power source. The rate of production is proportional to the current flow in the absence of parasitic reactions. For example, in a liquid alkaline water electrolysis cell, a direct current (DC) is passed between two electrodes in an aqueous electrolyte to split water (the reactant) into the component product gases: hydrogen and oxygen where the product gases evolve at the surfaces of the respective electrodes.
The achievement of a preselected level of production involves a trade-off between increasing the operating current density and increasing the number of the cells. Due to the physical nature of the electrolytic processes, the higher the current density, the higher the energy consumption per unit of production, and so the trade-off facing the cell designer is whether to bear the increase in capital cost of more cells or to pay higher operating costs through reduced energy efficiency. Increasing current density will also lead to more stressful operating conditions such as higher electrolyte temperature that will impose additional design requirements and added costs. In the final analysis, the trade-off is determined on a case-by-case basis by the external variables primarily driven by the cost of electricity.
In the conventional bipolar electrolyser a voltage is applied between the end electrode of a stack of electrode plates. One side of a plate acts as an anode and produces oxygen and the other side acts as a cathode producing hydrogen in the case of electrolysis. The key implications to this in terms of current flow is that the current flow is through the stack perpendicular to the plane of the electrode (the plane of the electrode defined by the gas evolving surfaces of the electrodes) and importantly that the current flow is contained within the cell stack. Current flows in the electrode from all edges of the electrode towards the centre of the electrode plate.
In the conventional mono-polar cell design presently in wide commercial use today, one cell or one array of (parallel) cells is contained within one functional electrolyser, or cell compartment, or individual tank. Therefore each cell is made up of an assembly of electrode pairs in a separate tank where each assembly of electrode pairs connected in parallel acts as a single electrode pair. The connection to the cell is through a limited area contact using an interconnecting bus bar such as that disclosed in Canadian Patent Number 302,737 issued to A. T. Stuart (1930). The current is taken from a portion of a cathode in one cell to the anode of an adjacent cell using point-to-point electrical connections using the above-mentioned bus bar assembly between the cell compartments. The current is usually taken off one electrode at several points and the connection made to the next electrode at several points by means of bolting, welding or similar types of connections and each connection must be able to pass significant current densities. Current flows from the point of connection over the area of the electrode. Current in the electrode flows only in the plane of the electrode. Current between cells occurs outside the nominal cell stack as each cell is in a separate tank. A drawback to such connections is that they are prone to oxidation and other types of degradation resulting in significant potential drops between cells which reduce the efficiency of the electrolyser.
Most filter press type electrolysers insulate the anodic and cathodic parts of the cell using a variety of materials which may include metals, plastics, rubbers, ceramics and various fibre based structures. In many cases, O-ring grooves are machined into frames or frames are moulded to allow O-rings to be inserted. Typically at least two different materials form the assembly necessary to enclose the electrodes in the cell and create channels for electrolyte circulation, reactant feed and product removal. One of the materials is, for example, a hard engineering plastic and the other a material soft enough to allow sealing to be achieved. In large bipolar filter press systems, cell stacks could be many tens of meters in length. Such systems require hard and rigid materials with compatible coefficients of thermal expansion and minimal temperature/pressure related creep.
It would be very advantageous to provide an electrochemical system which eliminates the need for external contacts connecting adjacent electrodes, which avoids the drawbacks to conventional monopolar and bipolar systems but incorporates the advantages of each into a system, and which reduces the number of components making up the system.
It is an object of the present invention to provide an electrochemical system having the compactness and low inter-cell resistance connections inherent in bi-polar like electrochemical systems. It is also an object of the present invention to provide compact electrochemical systems in a single stack configuration and in a multi-stack configuration.
Another object of the present invention is to provide an electrochemical system provided with a unitary one piece double electrode plate for supporting an anode located in one cell compartment and a cathode located in an adjacent cell compartment with the double electrode plate adapted for use in the aforementioned single stack and multi-stack electrochemical systems.
Another object of the present invention is to provide an electrochemical cell or electrochemical system having electrolyte circulation frames which can also serve as seals in order to avoid the need for separate gaskets.
Another object of the present invention is to provide multi-purpose rigid enclosures at one or both ends of the aforementioned electrochemical systems which are in flow communication with the cell compartments. The enclosures provide structural rigidity to the system in addition to acting as reservoirs for electrolyte and providing a location for separating reaction product from the electrolyte.
A further object of the present invention is to provide a double electrode plate including an electrically conducting frame for supporting two electrodes spaced from each other but both electrodes being electrically connected by the frame. The double electrode plate may be adapted for use as a component in electrochemical systems including but not limited to energy storage devices such as batteries, energy producing devices such as acid and/or alkaline fuel cells, electrochemical systems for various electrosynthesis reactions.
An advantage of the present invention is that it provides a mono-polar electrolyser configuration system having a compact cell design with reduced intercell resistance factors typically found in bi-polar electrolysers. Multiple stack electrolysers with power ratings of from less than one kW to several megawatts may be constructed in a single cell block in accordance with the present invention. Another advantage of the electrolysers of the present invention is that they do not require separate sealing gaskets as needed in conventional electrolysers.
The present invention relates to the design of an assembly of an electrochemical cell stack using a unitary electrode plate, hereinafter referred to as a double electrode plate (DEP), on which an anode and cathode are supported. The double electrode plate serves to electrically connect two adjacent cell compartments and wherein the current flow in the electrodes is parallel to the working face of the electrode plate. In the cell designs disclosed herein the monopolar cells are assembled as a contiguous stack of cells (cell stack) appearing similar to a filter press where the electrical connections between adjacent stacks are made using the double electrode plate.
In one aspect of the present invention there is provided a double electrode plate for supporting two electrodes. The double plate electrode comprises an electrically conducting frame having a first portion for supporting a first electrode and a second portion for supporting a second electrode. The first electrode and the second electrode are spaced apart and electrically connected by the electrically conducting frame.
In another aspect of the invention there is provided an electrochemical system comprising at least two cells, each cell defining an anolyte chamber and a catholyte chamber, and including at least an anode electrode adjacent to the anolyte chamber, and a cathode electrode adjacent to the catholyte chamber. The anolyte and catholyte chambers each include an entrance and exit. The electrochemical system includes at least one unitary one piece double electrode plate having an electrically conducting frame, the anode electrode in one of the at least two cells being supported on a first portion of the electrically conducting frame, and the cathode electrode in one of the other of the at least two cells being supported on a second portion of the electrically conducting frame spaced from the first portion.
The present invention provides an electrochemical system, comprising:
a) at least one cell stack including at least two cells in said at least one cell stack, each cell including an anode electrode and anolyte chamber adjacent to said anode electrode, a cathode electrode and catholyte chamber adjacent to said cathode electrode; and
b) a first rigid support member located at one end of said at least one cell stack, a second rigid support member located at the other end of said at least one cell stack with the first and second rigid support members being in structural engagement with said at least one cell stack, said first rigid support member defining a first enclosure for containing electrolyte therein, said second rigid support member defining a second enclosure for containing electrolyte therein, each of said first and second enclosures having an inlet and outlet and said anolyte chambers being in flow communication with the inlet and outlet of said first enclosure for recirculating electrolyte between said anolyte chambers and said first enclosure, and said catholyte chambers being in flow communication with the inlet and outlet of said second enclosure for recirculating electrolyte between said catholyte chambers and said second enclosure.
The present invention also provides an electrochemical system comprising at least one cell stack including at least two cells, each cell including a first conducting plate supporting an anode electrode and a second conducting plate supporting a cathode electrode The first and second conducting plates each include opposed peripheral surfaces. Each cell includes at least a first frame member sealingly engaged against one of the opposed peripheral surfaces of the first conducting plate defining an anolyte chamber. Each cell includes at least a second frame member sealingly engaged against one of the opposed peripheral surfaces of the second conducting plate defining a catholyte chamber. The first and second frame members are fabricated of a compressible elastomer-like material, and wherein the first and second frame members are compressed to form fluid tight seals when the electrochemical system is assembled.
The present invention provides a single stack electrochemical system. The system comprises n cells arranged serially in a cell stack wherein n is an integer number of cells greater than or equal to 2, each cell including at least one anode electrode and an anolyte chamber adjacent thereto and a cathode electrode and a catholyte chamber adjacent thereto. Two cells are located at opposed ends of the stack with one of the two cells including at least a contact anode electrode and the other of the two cells including at least a contact cathode electrode. The contact anode and contact cathode electrodes are adapted to be connected to a power supply. An insulating member for insulating adjacent cells in the stack is provided. The system includes at least nxe2x88x921 double electrode plates, each double electrode plate including a least an electrically conducting frame having a length and a width, and a first portion for supporting an anode electrode located in one of the cells and a second portion for supporting a cathode electrode located in an adjacent cell, and a web portion between the anode and cathode electrodes electrically connecting the electrodes. At least a portion of the web portion is located exterior to the cells containing the anode and cathode electrodes supported by the double electrode plate. The double electrode plate being folded substantially down a middle of the web portion, and when a voltage drop is developed between the two contact electrodes current is collected along the length of the electrically conducting frames and flows from cell to cell in a plane of the double electrode plates across the width of the electrically conducting frames and the anode and cathode electrodes.
The electrochemical cell stack designs disclosed herein utilizing the double electrode plates have a much higher ratio of surface area for electrochemical reaction per unit volume of electrolyser than conventional mono-polar designs based on individual cell tanks and therefore space requirements and system weight of embodiments of the electrolysis cell stacks are appreciably less than in current systems. Advantageously, the footprint or area occupied by the electrochemical system can be reduced further, without changing current density and with minimal impact to cell voltage by increasing the height of the electrode.
The electrochemical cells or systems using the double electrode plates constructed according to the present invention provide the advantages of compactness of size and low inter-cell resistance factor found in conventional bi-polar electrolysers and provide lower cost mono-polar electrolysers. Another advantage of the present invention is that using the multiple stack electrolyser (MSE) configuration, cell assemblies of 1 MW or larger can be constructed in a single cell block.